The present invention relates to telephone handsets and more particularly to the provision of a low frequency tone for signalling purposes from a handset.
With the increasing demands for more sophisticated facilities in telephone sets and the availability of cheap low power electronics to meet this need, many telephone administrations are considering the introduction of "on-hook" dialling facilities where the progress of calls is monitored through a separate small sound reproducer mounted in the telephone set. The handset remains on the cradle until the required connection is made.
The realisation of these facilities requires the reproduction and transmission into air at adequate levels say 75 dB spl at one meter--of a range of frequencies which can be typically as low as 50 Hz (U.K. Public Switched Network dialling tone) and which must also include a substantial part of the telecommunication speech band for the speech monitoring. Because of the restrictions on transducer size and available electrical drive parameters it is difficult to obtain adequate sound pressure levels at the low frequencies required for adequate tone monitoring (50 Hz, 400 Hz etc.) from currently available small transducers. This invention describes a means of overcoming this problem.